Kinetics study on hydrothermal dechlorination of poly(vinyl chloride) by in-situ sampling

Abstract

Hydrothermal pretreatment has been widely applied as an efficient way to remove chlorine from chlorinated organic wastes while real-time chlorine transformation behaviors and kinetics had not been fully understood. This work investigates on the real-time chlorine transformation characteristics and release kinetics during the hydrothermal dechlorination (HTD) of PVC to propose some powerful solutions to remove chlorine effectively. An online sampling system was designed and operated to collect the gaseous products in-situ. The chlorine in PVC was mainly removed in the form of HCl and then migrated into gaseous products and liquid products. The HTD temperature was the most important factor affecting chlorine removal, followed by residence time. The dechlorination efficiency (DE) was increased from 3.07% to 82.51% when the HTD temperature was risen from 175 °C to 280 °C at a residence time of 30 mins. The chlorine content of gaseous products was accordingly increased from 0.75% to 9.52%. The temperature above 200 °C caused the expansion of PVC and destruction of PVC structure. Besides, the release of HCl generated plenty of pores inside the PVC, enhancing the mass transfer and accelerating the PVC dechlorination. A maximum DE growth rate was observed at the HTD temperature range of 220∼235 °C. At a HTD temperature of 235 °C, the DE was increased from 72.60% to 84.78% when the residence time was prolonged from 15 mins to 60 mins. The kinetics analysis shows the HTD of PVC could be represented by the first-order reaction. According to the Arrhenius law, the apparent activation energy was 217.43 kJ/mol and the HTD reaction of PVC was a chemical reaction control process. This work could provide some fundamental data for the optimization design of hydrothermal pretreatment of chlorine-containing solid waste.